Single-frequency microchip lasers are already known and have a cavity length which is extremely short (usually measured in microns) such that only a single longitudinal mode falls under the gain curve of the gain medium when pumped by a laser diode. These microchip lasers have good inherent frequency stability, but where very high frequency stability is required (such as optical communications) they may require frequency locking to a stable reference interferometer in order to overcome frequency fluctuations arising from intensity variations in the pump diode output, acoustic noise, or mechanical vibration of the microchip structure.
Locking of laser frequencies is itself known by diverting at least some of the laser output through an electro-optic phase modulator driven at a modulation frequency and at a very low modulation index (of the order 0.1) to generate sidebands which are applied to the reference interferometer in order to provide an error signal when the sideband signal is reflected from the reference interferometer. The error signal is then fed back to the laser to strain a piezo-electric mirror mount and thereby correctively change the cavity length and hence the frequency output of the microchip laser.
Electro optic phase modulators utilize birefringent crystals such as ADP, KDP, LiNbO.sub.3 each of which is expensive to manufacture, requires very accurate angular orientation of its birefringent axes with respect to the optical axis of the laser system, in effect attenuates the available laser system output power, requires a polarized output from the laser, and a large drive power (typically of the order of 1 W), even for the low modulation index required because at the modulation frequencies concerned (10-100 MHz) such, crystals have significant capacitive impedance.